Under the trend of making automobiles light in weight, there has been an especially brisk demand for thin steel sheet with high strength having an excellent formability.
In addition, safety of automobiles has been thought to be important too and, accordingly, there has been a demand an improvement in crushing performance which is a yardstick for safety upon crash.
With regard to outer and inner panels for automobiles, cold rolled steel sheets are advantageous in terms of homogeneity of surface roughness and phosphatability.
Under such current circumstances, various cold rolled steel sheets with high strength have been developed already.
For example, in the Japanese Examined Patent Publication Hei-05/064215 and Laid-Open Patent Publication Hei-04/333524, there are disclosures on the method for the manufacture of high strength steel having a structure of ferrite containing not less than 3% of bainite and retained austenite (hereinafter, referred to as TRIP steel).
However, although the TRIP steel has a high elongation and a good formability (TS.times.El.gtoreq.22000 MPa. %), there is a problem that this steel does not satisfy the current requirement for severe crushing performance.
In addition, there is another problem that the work hardening (WH) at press formability and the bake hardening (BH) at painting and baking thereafter are as low as about 70 MPa.
When those work and bake hardenings (WH+BH) are low, there is a big disadvantage in terms of ensuring the strength after forming, painting and baking.
On the other hand, the so-called dual phase steel (hereinafter, referred to as DP steel) having a dual phase of ferrite and martensite is disclosed, for example, in the Japanese Laid-Open Patent Publication Hei-09/111396 as a high strength steel sheet having an excellent crushing performance.
However, although the DP steel has an excellent crushing performance, its elongation is not sufficient and there is a problem in formability.
As mentioned above, no cold rolled steel sheet which satisfies both requirements of sufficient formability and severe safety standard has been available at present and, therefore, there has been a demand for developing it.